An image forming apparatus comprises an image sensor configured to extend in a horizontal scanning direction, a carriage, a guide section and a reading control section. The carriage configured to move in a vertical scanning direction orthogonal to the horizontal scanning direction to change a reading position of a document. The guide section configured to have a guide surface for guiding the document to the document glass, and a white part, a first colorizing part and a second colorizing part which are formed on the guide surface in line in the vertical scanning direction. The reading control section configured to move the carriage to align the reading position in the vertical scanning direction with one of the white part, the first colorizing part and the second colorizing part and execute a reading processing.